The promise of abundant energy has inspired many approaches to controlled thermonuclear fusion. Pellets, ignited one per second, could be the energy source for power plants producing billions of watts of electrical power. To achieve ignition, one would have to deliver about 1 megajoule (MJ) of energy to a deuterium-tritium (DT) pellet in about 10 nanoseconds (ns). Candidate igniters have included laser, electron, and heavy- and light-ion beams.
Another approach would be to ignite a DT pellet with the impact of a projectile weighing about 0.1 gram. Such a projectile could be accelerated to hyperveloicty (150 km/s or more) by a magnetic accelerator. The advantage of the use of a projectile is that the energy would be concentrated into a small volume. If the projectile moves rapidly enough to contain the energy, then it would be easy to deliver the energy in the required 10 ns by making the projectile short enough.
The present invention relates to an electromagnetic railgun as a projectile launch device.
In order to better understand the present invention, the operating principles and limitations of a prior art electromagnetic railgun system, as exemplified in FIGS. 1-3, should first be considered.
A railgun accelerator 10, as shown in FIG. 1, is a linear dc motor consisting of a pair of rigid, electrically- and field-conducting rails 11 and 12 and a movable conducting armature 13. Basically, the armature 13 is accelerated along the rails as a result of the Lorentz force produced by the current, I, from a primary energy-storage device (PESD) 14, in the armature 13 interacting with the magnetic field B produced by the current in the rails 11 and 12. The armature 13 acts upon the rear of projectile 15 to accelerate it down the rails 11 and 12.
Preferably, a plasma arc is used as the armature 13, the arc being produced between the rails by the PESD 14. Typically, a rail gun assembly may be as shown in FIG. 2, with a dielectric 16 serving to maintain the rail position, to form with rails 11 and 12 the bore 17 of the railgun, and to confine the plasma arc armature 13 behind the projectile 15. A jacket 18, of steel for example, serves as the supporting barrel of the railgun.
A typical rail gun system, as shown in FIG. 3, functions as follows. A PESD 14, such as a capacitor bank or homopolar generator, is used to generate a current in the storage inductor 21 after switch 22 is closed. When the desired, usually maximum, current is established in inductor 21, switch 24 is closed to isolate the PESD 14 from the circuit. At this time, and if the PESD 14 is a homopolar generator, shuttle switch 26, such as a sliding multifingered conductor initially bridging between the two busbars 27 and 28, is moved across the breech end 29 of the railgun 10 to bridge between busbars 28 and 30. Rails 11 and 12 are electrically connected to busbars 28 and 30, respectively. As the shuttle switch 26 moves across the breech end of the rail gun, out of contact with busbar 28 and into contact with busbar 29, fusible wire 31, connected between the rails 11 and 12 will initially conduct current but will quickly vaporize and establish the plasma arc. (If the PESD is a capacitor bank, busbars 27 and 30 are shorted together and shuttle switch 26 is not needed).
The plasma arc 13 and projectile 15 will then accelerate along the rails 11 and 12. Prior to the arc exiting the discharge end 32 of the railgun, crowbar switch 33 is closed to extinguish the plasma arc and avoid spurious arcing.
The use of a plasma arc as an armature 13 for accelerating the projectile has several advantages over a sliding metallic conductor. First, the plasma arc easily maintains contact with the rails. Secondly, a conducting metallic armature resistively melts. Thirdly, a sliding metal contact experiences a large erosive drag force.
Acceleration, a, of the projectile 15 is given by ##EQU1## where I is the current in the arc, w is the rail spacing, B is the magnetic-field intensity in the region of the arc, m is the mass of the projectile, and L.sub.1 is the inductance per unit length of the rail gun.
The projectile velocity, v, is given by EQU v=.intg.adt
wherein t is time, and the projectile position z, is given by EQU z=.intg.vdt.
At high current density, the plasma-arc voltage, V.sub.A, is nearly independent of the arc current and is equal to about 200 v.
The voltage, V.sub.I, resulting from the time variation of the current and inductance, L, of the railgun, is given by ##EQU2##
Since L=L.sub.1 z, then (dL/dt)=L.sub.1 v.
The voltage, V.sub.R, along the two rails is given by EQU V.sub.R =2 IRdz
where R is the resistance of each rail.
Using Kirchhoff's law, ##EQU3## from which the current and voltages are calculated. (Stray circuit resistance and inductance are included in R.sub.o and L.sub.o of resistance 23 and inductance 21, respectively.
The following equation may be used to calculate the distribution of energy throughout the projectile's acceleration. The instananeous energy, E.sub.c, in the storage coil 21 is ##EQU4##
The inductive energy, E.sub.I, between the rails is ##EQU5##
The energy loss, E.sub.A, in the plasma arc is EQU E.sub.A =.intg.V.sub.A Idt.
The energy loss, E.sub.R, in the fixed elements and rails is given by EQU E.sub.R =.intg.I.sup.2 R.sub.o dt+2.intg.I.sup.2 Rdt
The instantaneous kinetic energy, E.sub.p, of the projectile is EQU E.sub.p =(mv.sup.2 /2)
Single-stage railguns as described above have been used to accelerate projectiles to velocities of up to 10 km/s. Higher velocities are obtainable, but the design and operation of a railgun is restricted by several practical considerations.
In order to prevent rail melting, or undue loss of rail strength from high temperature, the perimeter current density for a copper rail system initially at room temperature might be limited to 43 kA/mm for a single launch. If the system is initially at liquid nitrogen temperature, the perimeter current density might be limited to 75 kA/mm.
The magnetic pressure on the rails is a function of the current per mm of rail spacing. If a hardened steel rail is used for strength, with copper plating for electrical efficiency, then in order for the magnetic pressure forces to remain below the yield point of hardened steel with a typical elastic strength of 0.7 GPa (10.sup.5 psi), the current must remain less than 75 kA/mm of rail spacing.
In order to protect against destructive acceleration and maintain the mechanical integrity of a square-bore projectile having a typical elastic strength of 1.4 GPa (2.times.10.sup.5 psi), the current must remain less than 81 kA/mm of rail spacing.
Because launch performance improves with current and because current per unit spacing and current per unit perimeters have limits as set forth above, it is desirable to maximize rail spacing and rail perimeters. The perimeter can be increased indefinitely on the outside portion of the rails, but the rail spacing governs the bore size (assumed to be square). The aspect ratio, A.sub.R, defined as the ratio of the length to the height and width of the projectile must remain greater than 0.5 to maintain dynamic stability. Hence, increasing the bore results in a longer, larger, and more massive projectile, which in turn requires more input energy and a longer accelerator. Accordingly, the choice of bore size is a compromise between competing factors that vary with a specific application.
A spurious arc discharge between the rails, other than the arc driving the projectile, will divert some or all of the remaining energy delivered to the rails. The inductive voltage appears across the rails immediately behind the driving arc. The resistive voltage occurs along the rails from the arc toward the breech of the railgun where the total voltage appears. The breakdown voltage is a function of rail spacing and magnetic field strength, and thus establishes the smallest bore that can be used without spurious arc.
In addition to the above considerations, the performance of a railgun launch is limited by the amount of energy available to it. Maximum available energy loss incurred in transfers from the PESD to the storage inductor and then to the railgun.
Based upon the above considerations a railgun accelerator system as shown in FIGS. 1-3 can be designed to accelerate a 0.1 payload to a velocity of 150 km/s. The projectile 15 comprises a sabot, or carrier, in which the payload is mounted to permit its launching and the payload. The sabot is typically a graphite composite.
As brought out above, a higher current leads to a shorter accelerator and lower energy loss. The limit on current per unit rail spacing (75 kA/mm) requires a larger bore for higher current. However, as the bore increases, the mass of the sabot increases, requiring more energy for its launching. As a consequence, even though a larger bore permits higher current and hence acceleration force, a small bore is superior because of the smaller sabot mass and resulting higher velocity. The breakdown voltage establishes in the presently described system, a 6.7 mm bore as the smallest that can be used. To provide a safety margin, the minimum rail spacing can be about 10 mm.
Accordingly, the rails 11 and 12 should have a height of 10 mm and be spaced 10 mm apart to provide a square bore. To prevent rail melting, the rails should have a 40 mm perimeter. A square bore of 10 mm per side will require the sabot to have a length of 5 mm and consequent mass of 1.13 g. With a 0.1 g payload, the projectile mass will thus be 1.23 g.
To achieve a launch velocity of 150 km/s, and with a current limit of 750 kA, a minimum of 52 MJ of initial energy in the storage inductor 21 would be required. Since PESD energy must be greater by the amount lost in charging the storage indicator, and with expectable 85% efficiency, the PESD energy would be about 60 MJ.
If there is enough stored energy to maintain a constant maximum current of 750 kA throughout acceleration, a railgun length of at least 115 m is required to achieve the desired velocity. When the stored energy is not adequate to maintain constant maximum current, the length of the accelerator must be increased.
The efficiency of converting the initial energy stored in the inductor into kinetic energy of the payload can be 2% at 150 km/s. If the kinetic energy of the sabot mass could also be used as a payload, the efficiency would be about 25%. The efficiency will vary somewhat depending upon whether the recoverable inductive energy in the railgun is recovered for use in the next launch or not.
Although a railgun system as described above can obtain the desired launching speed of the payload, it has several significant disadvantages. Approximately half of the energy stored in the inductor is lost in resistive heating of the rails. A very large capacity PESD would be required to furnish the energy without current decay. Current decay would require a longer railgun, which, in turn, would increase the resistive losses and reduce efficiency.
The present invention is directed to overcoming one or more of the problems as set forth above.